Method for manufacturing a monolithic building material with carbonatable granular material and for limiting leaching of corrosive anions and heavy metals from the latter

Abstract

Method for manufacturing a monolithic building material from a combined granular material containing carbonatable material, derived from, for example, steel manufacturing, wherein the combined granular material is compacted into a shaped unit, wherein a CO2-containing gas is brought into contact with the carbonatable material of the shaped unit and reacts with the latter, thereby forming carbonates and bonding the granular material to form the monolithic building material, wherein, before bringing the carbonatable material into contact with the CCh-containing gas, water-soluble magnesium salts, namely magnesium nitrate, is added to the combined granular material.

Description

[0001] Method for manufacturing a monolithic building material with carbonatable granular material and for limiting leaching of corrosive anions and heavy metals from the latter

[0002] The invention relates to a method for manufacturing a monolithic building material from granular material containing carbonatable material, wherein, in a moulding phase, the granular material is compacted into a shaped unit, wherein, in a carbonation phase, a CCh-containing gas is brought into contact with the carbonatable material of the shaped unit and reacts with the latter, thereby forming carbonates and bonding the granular material to create the monolithic building material.

[0003] The granular material containing carbonatable material can be obtained at least in part from steel slag. This carbonatable material contains, for example, calcium hydroxide, which reacts with carbon dioxide from the CO2- containing gas to form calcium carbonate, thereby binding the material.

[0004] According to this method, the building material is not obtained by using a hydraulic binder whereby hardening occurs through reaction with water, as is the case with cement-bound building materials, for example. It is preferably a non-cement-bound monolithic building material. The material is therefore bound by the formation of carbonates using CCh-containing gas. Carbonation with a CO2- containing gas is the process in which carbonatable material is exposed to CO2, whereby this material reacts with CO2 and forms carbonates.

[0005] Slags originating from steel manufacturing mainly consist of calcium compounds such as merwinite, bredigite, portlandite, beta and gamma dicalcium silicate, akermanite and srebrodolskite, which are carbonatable to varying degrees. This property makes the aforesaid slags potentially powerful binders that can also permanently store CO2 during the formation of calcium carbonates and offer good mechanical performance. It has incited researchers and manufacturers to crush these slags for use as binders. For example, press-moulded monoliths are manufactured by moulding this binder in a press, possibly together with other granular material, and then allowing this monolith to harden in the presence of CO2 through carbonation. The hardening and strength development of such monoliths has already been extensively studied in order to optimise this process of accelerated mineral carbonation.

[0006] Nevertheless, slags from steel manufacturing inherently contain significant concentrations of corrosive anions, such as fluorine (F) and bromide (Br), as well as heavy metals such as chromium (Cr), vanadium (V), and molybdenum (Mo). These elements are added to the steel melt to improve process efficiency and / or to impart specific properties to the resulting steel or stainless steel. Consequently, these elements become integral components of the slag produced during steelmaking. When such steel slags are used to produce monolithic building materials such as paving stones, bricks and building blocks through mineral carbonation, the potential leaching of these hazardous elements becomes a critical environmental concern. Leaching of F, Cr, V and Mo during the life and / or second life of these building materials compromises their sustainable use and / or reuse, respectively. If released into the environment, these corrosive anions and heavy metals may have detrimental effects on ecosystems, water quality and public health. Therefore, addressing this challenge is essential for the acceptability of building materials derived from steel slag.

[0007] Patent document BE1010700A5 describes a method for processing stainless steel slag wherein this slag is broken and crushed and then bound with a hydraulic binder, such as cement, into a dimensionally stable monolithic mass, thereby preventing any leaching of chromium oxide, nickel oxide and fluorides.

[0008] In patent document US8,623,134B2, additives such as calcium and / or magnesium acetate and / or calcium chloride are added to promote the hydration of calcium and magnesium oxides from granular steel slag material, thus promoting carbonation in its turn. This results in carbonated aggregates from the granular material that can be used in mortar or concrete.

[0009] European patents EP2276714B1 and EP3186210B1 describe methods for manufacturing a non-cement-bound monolithic building material obtained from carbonatable granular material derived from steel slag. In these patents, the granular material from crushed steel slag is compacted into shape and then exposed to CO2 in gaseous form to bind the granular material. Patent application CN118184280A describes the use of soluble magnesium salts such as magnesium chloride and magnesium sulphate to promote the formation of aragonite in the production of carbonated cellular concrete with steel slag.

[0010] Patent application WO2024 / 256242 describes a method for producing a monolithic building material from granular material containing carbonatable material that is compacted and then exposed to a CCh-containing gas so that the granular material is bound by carbonation, creating the monolithic building material. A water-soluble magnesium salt is added among other things to efficiently immobilise fluorine during carbonation. Magnesium chloride is preferably used for this purpose, but magnesium sulphate, magnesium citrate or magnesium nitrate are also suggested.

[0011] When these monolithic building materials are used as facing bricks to replace traditional clay bricks, it may be important for them to have similar properties as the traditional bricks, such as density, porosity, initial water absorption and permeability. However, if a higher porosity, water absorption and permeability is wanted, it may increase the risk of undesirable leaching.

[0012] The invention aims to remedy these disadvantages by proposing a method in which the monolithic building material is obtained from granular material containing carbonatable material by means of carbonation with a CO2- containing gas, thereby limiting or preventing leaching of corrosive anions and heavy metals from this building material. This is particularly the case when the carbonatable material originates from, for example, steel slag such as stainless-steel slag.

[0013] To this end, the invention proposes a method, as claimed in the appended claims, in which water-soluble magnesium salts are added to the granular material before bringing the carbonatable material into contact with the CO2- containing gas.

[0014] These water-soluble salts have a water solubility which preferably exceeds 40 g per 100 ml, and in particular exceeds 50 g per 100 ml under atmospheric pressure and at a temperature between 10°C and 70°C. The soluble magnesium salts are effectively selected from magnesium nitrate and / or hydrated forms thereof. These hydrated forms may be, for example, magnesium nitrate hexahydrate.

[0015] During the moulding phase, the combined granular material is compacted using a compaction pressure applied to the combined granular material of 12 MPa at the most, preferably 9 MPa at the most, and in particular 8 MPa at the most.

[0016] Advantageously, magnesium nitrate is added in a quantity of 0.29 to 5 mass% of the combined granular material, preferably 0.29 to 2 mass%, more preferably 0.29 to 1.16 mass%, and in particular 0.29 to 0.87 mass%. Naturally, these quantities also include equivalent quantities of hydrated forms of magnesium nitrate.

[0017] In an interesting manner, soluble magnesium salts, specifically magnesium nitrate, are added in quantities exceeding 0.29 mass% of the combined granular material.

[0018] In another interesting manner, soluble magnesium salts, i.e. magnesium nitrate, are added in quantities exceeding 0.5 mass% of the combined granular material, preferably exceeding 1 mass%.

[0019] The carbonatable material may possibly originate from blast furnace slag, metal slag, fly ash and / or bottom ash, in particular crushed and / or ground fractions thereof, and preferably contains calcium oxide, calcium hydroxide, magnesium oxide and / or magnesium hydroxide.

[0020] In a very advantageous manner, the carbonatable material is obtained from slag originating from steel manufacturing, in particular stainless- steel manufacturing.

[0021] Preferably, before exposing the carbonatable material to the CO2- containing gas, the combined granular material’s moisture content is adjusted to 5 to 12 mass%, preferably 7 to 10 mass%, in particular 7.5 to 8.5 mass%. The carbonatable material may also be dried first to achieve a lower moisture content and then be re-moistened. Preferably, the combined granular material is composed of at least 50 mass%, preferably at least 60 mass% of granules having a grain size of less than 0.25 mm.

[0022] Preferably, the combined granular material is composed of at least 40 mass%, preferably at least 45 mass% of granules having a grain size of less than 0.125 mm.

[0023] Preferably, the combined granular material is composed of granules having a grain size of up to 1.5 mm.

[0024] In an extremely advantageous manner, the combined granular material is composed of at least 50 mass%, preferably at least 60 mass%, and in particular at least 70 mass% of granules obtained from slag originating from steel manufacturing, in particular stainless-steel manufacturing.

[0025] Preferably, the combined granular material is compacted in the moulding phase before the carbonatable material is exposed to the CCh-containing gas, applying a compaction pressure on the combined granular material of at least 4 MPa.

[0026] Preferably, the combined granular material is compacted in the moulding phase before the carbonatable material is exposed to the CCh-containing gas, applying a compaction pressure on the combined granular material of 4 to 9 MPa, preferably 4 to 8 MPa, and in particular, 4.5 to 8 MPa.

[0027] Preferably, the CCh-containing gas contains at least 5 volume% CO2, preferably at least 10 volume% CO2, and in particular at least 20 volume% CO2.

[0028] In a simple way, the magnesium nitrate is added as an aqueous solution.

[0029] Other particularities and advantages of the invention will become apparent from the following description of specific embodiments of the method according to the invention; this description is given by way of example only and does not limit the scope of the claimed protection in any way.

[0030] The invention generally relates to a method for manufacturing a monolithic building material by means of what is called mineral carbonation of a granular material containing carbonatable material derived from steel slag. The monolithic building material can be used, for example, to create facing bricks and / or facing brick strips. During mineral carbonation, gaseous CO2 reacts with alkaline minerals, such as calcium and magnesium oxide, forming carbonates and thus binding the granular material. Steel slag is rich in calcium compounds, such as calcium silicates and calcium oxides, that are carbonatable and form calcium carbonates when CO2 is added. To this end, steel slag is ground into fine granular material or powder. Preferably, different fractions with different grain sizes are obtained, such as fractions with grain sizes smaller than or equal to 1.5 mm, 500 pm, 250 pm, and / or 125 pm.

[0031] The granular material derived from steel slag is preferably composed of different fractions and possibly some other granular material in order to obtain the desired grain size distribution. If necessary, the material is moistened to achieve the desired moisture content.

[0032] The carbonatable material preferably contains calcium oxide and calcium hydroxide. It may also contain magnesium oxide and magnesium hydroxide. Examples of this material are portlandite (Ca(OH)2), quicklime (CaO), periclase (MgO), and brucite (Mg(0H)2). This material can be obtained from metal slag, for example, in particular from steel slag originating from stainless-steel manufacturing.

[0033] To obtain a suitable granular material, metal slag can be crushed, allowing also the metals present to be at least partially removed and reused. From the crushed material, a fine fraction of granules can be separated with a grain size smaller than 4 mm, smaller than 1.5 mm, and / or mainly smaller than 0.5 mm. This fraction can be further separated into different sub-fractions with different grain size distributions. These different sub-fractions can then be recombined in the desired proportions to obtain the desired grain size distribution.

[0034] The granules obtained from steel slag typically contain 40 to 75 mass% of carbonatable material and may, for example, consist mainly of CaO, in quantities of 30 to 70 mass%, and to a lesser extent of MgO, in quantities of up to 15 mass%. In addition, these granules may contain non-carbonatable material, such as SiO2, in quantities of up to 30 mass%. Other components may also be present in smaller quantities, such as, for example, AI2O3, Fe20a, MnO, TiCh and possibly also F’, Br, Cr, V, Mo.

[0035] The quantity of carbonatable material is preferably at least 40 mass% and can of course be as high as possible.

[0036] The granular material thus obtained, containing granules with carbonatable material, can then be further mixed with non-carbonatable granular material, such as, for example, quartz sand or silica sand.

[0037] This combined material is moulded into shaped units with the desired dimensions, density and porosity using a press. The density and porosity of the shaped units are determined by factors such as grain size distribution and / or degree of compression. These properties may be important for the building material’s use as a facing brick and / or paving stone, for example.

[0038] Thus, during compression, the compaction pressure on the granular material should be no more than 12 MPa, preferably no more than 9 MPa, and in particular no more than 8 MPa to ensure that the aforementioned properties, such as density and porosity, and / or initial water absorption and permeability, are not negatively affected when using the building material as a facing brick and / or paving stone, for example. Compression is also applied to ensure that the shaped unit has sufficient strength. A minimum compaction pressure of 3 MPa, preferably 4 MPa, and in particular 4.5 MPa, is preferably applied to obtain optimal compressive strength of the building material after carbonation.

[0039] The shaped unit is then exposed to CCh-containing gas during a carbonation phase. During this phase, the carbonatable material reacts with CO2, forming carbonates and binding the granular material together to create the monolithic building material. Absorption of CO2 and formation of carbonates will further change the density and porosity of the building material.

[0040] Desired typical values that can be obtained are, for example, a density of 1,800 to 2,400 kg / m3, porosity of 12 to 38 volume%, water absorption of 6 to 9 mass%, and permeability of 1 * 10'9to 3 * 10'9cm2.

[0041] The granular material derived from steel slag may therefore contain corrosive anions, such as fluoride (F) and bromide (Br), as well as heavy metals, such as chromium (Cr), vanadium (V) and molybdenum (Mo), which might leach from the resulting monolithic building material. The binding of the granular material through the formation of carbonates largely prevents such leaching.

[0042] Leaching may be problematic when using the monolithic building material in its first life, or when processing it in a second life cycle, for example after crushing or otherwise.

[0043] In order to further prevent leaching, magnesium nitrate is added to the granular material as a water-soluble magnesium salt before the carbonatable material is exposed to the CCh-containing gas. Magnesium nitrate may, of course, be used in its hydrated form, such as magnesium nitrate hexahydrate.

[0044] These soluble magnesium salts react with corrosive anions, such as fluoride, whereby they can form stable compounds, such as magnesium fluoride (MgF2). In addition, these soluble magnesium salts also have a positive effect on the prevention of heavy metal leaching, including Cr, V and Mo. The magnesium salts facilitate the binding of heavy metals by promoting the formation of stable compounds.

[0045] The water solubility of these magnesium salts ensures a better dispersion of magnesium within the matrix of the shaped unit, effectively increasing its interaction with potentially leaching corrosive anions and / or heavy metals.

[0046] Experimental results show that the addition of 1 to 2 mass% of such magnesium salts, in particular magnesium nitrate, to the combined granular material, prior to the carbonation phase with the addition of CO2, produces monolithic building materials that effectively reduce the leaching of F, V, Cr and Mo.

[0047] According to the present invention, magnesium nitrate or a hydrated form thereof is used as water-soluble magnesium salt. Magnesium acetate as a water-soluble magnesium salt also has a beneficial effect on leaching, but a major disadvantage is that acetate appears to promote undesirable efflorescence of salts and minerals. This is particularly detrimental when using these press-moulded carbonate-bonded building materials for facades and / or paving.

[0048] The following techniques and methods may be used to determine grain size, grain size distribution, CO2 absorption, moisture content, compressive strength, water absorption, frost resistance and leaching. Determination of grain size and grain size distribution

[0049] Grain size and grain size distribution are determined by dry sieve analysis. A sieving method in accordance with standard NBN EN 933-1 :2012 (publication date 02 / 2012) is used to this end. The quantities are expressed as a percentage by mass of the total dry mass.

[0050] Determination of CO2 absorption

[0051] The quantity of CO2 absorbed by the carbonatable material after carbonation is determined by monitoring the weight loss of the dried material during heat treatment at 350°C, 550°C and 950°C. The absorption of CO2 in the carbonation phase can be determined by measuring the difference in weight loss of the dried granular starting material before the carbonation phase and of the dried bricks of the building material after the carbonation phase.

[0052] Determination of moisture content

[0053] The moisture content of the material is determined by weighing a quantity of material and then weighing it again after drying. Drying can take place at a temperature of, for example, 105°C. The moisture content is the difference between the original and dried weights and is expressed as a percentage by mass of the original weight. The material is considered dry when the weight deviates by no more than 0.1%.

[0054] Determination of frost resistance

[0055] The frost resistance of the manufactured monolithic building material is determined in accordance with standard NEN-EN 772-22:2019 (publication date 01 / 2019). To this end, panels are manufactured using bricks made from this building material which are subjected to alternating freeze-thaw cycles, after which any damage is assessed. The bricks to be assessed are subjected to 100 freeze-thaw cycles.

[0056] Determination of compressive strength and water absorption

[0057] The obtained monolithic building materials are subjected to standardised tests in accordance with standard EN 771-1 :2011 to determine the characteristics of the building material, such as compressive strength and water absorption.

[0058] Determination of leaching To determine leaching, the obtained building materials are subjected to a standardised batch leaching test in accordance with standard EN 12457-2:2002. For this test, the obtained monolithic building materials are crushed into granules with a grain size of less than 4 mm and subjected to a shake-leaching test at a liquid-to-solid ratio of 10 litres per kilogram for 24 hours. The leaching of Cr, Mo, V and F is thereby monitored. The eluent from the leaching tests is analysed using atomic emission spectrometry (ICP-AES) in accordance with standard NEN 6966:2005 to determine the concentrations of Cr, Mo and V, and using potentiometry in accordance with standard NEN 6578:2011 to determine the concentration of F.

[0059] Experimental results

[0060] In a series of experiments 1 to 10, monolithic building materials were manufactured with varying granular compositions, as is also shown in Table 1. Two types of steel slag were used for this purpose which, on the one hand, originated from the production of stainless steel in a basic oxygen furnace (BOF) according to the oxy-steel process, known as BOF slag, and, on the other hand, from the production of steel in an electric-arc furnace (EAF), known as EAF slag.

[0061] Both types of slag were crushed and ground to obtain two fractions, namely a fine fraction with a grain size of less than 125 pm and a coarser fraction with a grain size ranging from 125 pm to 1,500 pm.

[0062] The fractions were mixed in the desired proportions, as shown in Table 1, together with the desired quantity of silica sand, with a grain size ranging from 0 pm to 500 pm, water and soluble magnesium salts.

[0063] This resulted in a combined granular material that was compacted and moulded into shaped units. Compression was performed using a hydraulic press with a compaction pressure of approximately 8 MPa. This produced a shaped unit measuring 240 mm x 50 mm x 72 mm with a density of 2,100 to 2,200 kg / m3. The composition and compaction pressure can be modified depending on the desired properties of the building materials to be obtained. For instance, the moisture content, grain size distribution and compaction pressure influence permeability, density and porosity, thereby affecting compressive strength and water absorption. The shaped units were then exposed to CO2 gas in a carbonation phase. This took place in a sealed reaction chamber at atmospheric pressure, a temperature of approximately 40°C, a relative humidity of 50% to 70%, and a CO2 concentration of 30% for 48 hours. Of course, these reaction conditions can be modified or optimised, for example, depending on the desired properties of the building materials to be obtained or to accelerate the carbonation process further. Thus, different reaction steps can be incorporated, or a higher ambient pressure, temperature, humidity and / or CO2 concentration can be applied.

[0064] The characteristics of the obtained monolithic building materials that are important for their use as facing bricks, for example, are determined to check them. These characteristics include among others compressive strength, water absorption and frost resistance. This makes it possible to verify or rule out whether the addition of soluble magnesium salts has any negative effects on the building materials obtained.

[0065] Furthermore, any leaching of corrosive anions and heavy metals from the obtained monolithic building materials is determined in order to assess the influence of adding soluble magnesium salts to the combined granular material.

[0066] In experiments 1 to 4 and 7, the granular combined was formulated with granular material obtained from EAF slag. In the leaching test, concentrations of F’’ Cr, Mo and V of 62 mg / kg, 1.2 mg / kg, 0.75 mg / kg and 1.7 mg / kg, respectively, were observed.

[0067] When magnesium nitrate hexahydrate was dissolved in the water to be added in a quantity of 1.5% by mass in experiment 4, the concentration of these components in the leaching test dropped to 48%, 31%, 15% and 53%, respectively.

[0068] In experiments 5 and 6, the granular combined was formulated using granular material obtained from BOF slag. The leaching test revealed concentrations of F’, Cr, Mo and V of 176 mg / kg, 0.44 mg / kg, 2.1 mg / kg and 1.3 mg / kg, respectively.

[0069] When 1% by mass of magnesium nitrate hexahydrate was dissolved in the water to be added in experiment 6, the concentrations of these components in the leaching test dropped to 57%, 64%, 43% and 46%, respectively. In a series of experiments 11 to 14 similar to the above experiments 1 to 4 and 7, but with a carbonation phase over a period of 4 days and using granular material from other similar steel slags from stainless-steel manufacturing, the effect of higher magnesium nitrate doses was investigated up to a quantity of 3.64% by mass of magnesium nitrate equivalent or 1.00% by mass of MgO equivalent, as shown in Table 2.

[0070] These experiments show that higher quantities of magnesium nitrate do not further reduce the leaching of F’, Cr, Mo and V. However, the compressive strength and CO2 absorption do decrease significantly at higher magnesium nitrate dosages.

[0071] Consequently, we can conclude that adding soluble magnesium salts such as magnesium nitrate or hydrates thereof significantly contributes to preventing any potential leaching of corrosive anions and heavy metals from building materials manufactured by the process of mineral carbonation from steel slag containing these corrosive anions and heavy metals. According to the invention, the addition of magnesium nitrate in a quantity of 0.29 to 1.16 mass% of the combined granular material, or of hydrates thereof in an equivalent quantity, effectively contributes to counteracting such leaching without impairing characteristics such as compressive strength, water absorption and / or CO2 absorption. Higher doses of magnesium nitrate do not result in a significantly better effect with regard to leaching, but should be avoided due to, among other things, the lower compressive strength and CO2 absorption obtained with these higher doses.

[0072] Table 1. Experimental results of a leaching test with monolithic building material obtained from EAF slag (experiments 1 to 4) and from BOF slag (experiments 5 and 6) while adding soluble magnesium salts in the form of magnesium nitrate hexahydrate (experiments 2, 3, 4 and 6).

[0073] Experiment

[0074] 1 2 3 4 7 5 6

[0075] Composition of the combined granular material (mass%*)

[0076] - stainless steel slag (<125 gm) 45 45 45 45 45 45 45

[0077] - stainless steel slag (125 / 1.500 gm) 28 28 28 28 28 28 28

[0078] - sand (0 / 500 gm) 18 18 18 18 18 18 18

[0079] - soluble magnesium salts: „ „ - . „ . - , „ „ . „ magnesium nitrate hexahydrate

[0080] (magnesium nitrate equivalent) (0.00) (0.29) (0.58) (0.87) (1.16) (0,00) (0.58)

[0081] (MgO equivalent) (0.00) (0.08) (0.16) (0.24) (0.31) (0,00) (0.16)

[0082] - water 9.0 8.5 8.0 7.5 7.0 9.0 8.0

[0083] Building material characteristics (EN 771 - 1 )

[0084] - compressive strength (MPa) 34.1 27.3 28.8 32.2 23.3 31.2 29.5

[0085] - water absorption (mass%) 7.2 6.6 5.7 6.1 5.6 8.3 6.8

[0086] - frost resistance +** + + + + + + Leaching (EN 12475-2) (mg / kg)

[0087] - F- 62 55 41 32 33 176 75

[0088] - Cr 1.20 0.74 0.84 0.83 0.99 0.44 0.16

[0089] - Mo 0.75 0.64 0.64 0.64 0.74 2.1 1.2

[0090] - V 1.7 1.3 1.0 0.8 0.9 1.3 0.7

[0091] *relative to the dry mass;

[0092] ** + no observable frost damage after freeze-thaw cycles. Table 2. Experimental results of a leaching test with monolithic building material obtained from EAF slag with a combined granular material composition similar to that in Table 1 (experiments 1 to 4 and 7) with the addition of magnesium nitrate (experiments 11 to 14).

[0093] Experiment

[0094] 11 12 13 14

[0095] Soluble magnesium salts: magnesium nitrate dosage (mass%*)

[0096] Magnesium nitrate hexahydrate1. , , . „ , _

[0097] . . 1.4 5.2 4.0 0.3 equivalent

[0098] (magnesium nitrate equivalent) (0.81) (1.85) (2.78) (3.64)

[0099] (MgO equivalent) (0.22) (0.50) (0.75) (1.00)

[0100] Building material characteristics (EN 771-1)

[0101] - compressive strength (MPa) 35.0 36.8 32.8 21.7

[0102] - water absorption (mass%) 9.5 8.8 9.2 10.0

[0103] - CO2 absorption (mass%) 7.6 6.4 5.1 4.3 Leaching (EN 12475-2) (mg / kg)

[0104] - F- 27.60 19.41 16.24 16.17

[0105] - Cr 0.27 0.20 0.17 0.15

[0106] - Mo 0.53 0.56 0.73 0.90

[0107] - V 1.20 0.68 0.33 0.19

[0108] * relative to dry mass

Claims

Claims1. Method for manufacturing a monolithic building material from a combined granular material containing carbonatable material, wherein, in a moulding phase, the combined granular material is compacted into a shaped unit, wherein, in a carbonation phase, a CCh-containing gas is brought into contact with the carbonatable material of the shaped unit and reacts with the latter, thereby forming carbonates and bonding the granular material to form the monolithic building material, characterised in that, before bringing the carbonatable material into contact with the C Ch-containing gas, magnesium nitrate is added to the combined granular material; and wherein, in the moulding phase, the combined granular material is compacted with a compaction pressure applied to the combined granular material of up to 12 MPa, preferably up to 9 MPa, and in particular up to 8 MPa.

2. Method according to claim 1, wherein, before exposing the carbonatable material to the CCh-containing gas, magnesium nitrate is added to the combined granular material in a quantity of 0.29 to 5 mass% of the combined granular material, preferably 0.29 to 2 mass%, and in particular, 0.29 to 1.16 mass%.

3. Method according to claim 1 or 2, wherein, before bringing the carbonatable material into contact with the CCh-containing gas, magnesium nitrate is added to the combined granular material in a quantity of 0.29 to 0.87 mass% of the combined granular material.

4. Method according to any of claims 1 to 3, wherein the carbonatable material is obtained from slag originating from steel manufacturing, in particular stainless-steel manufacturing.

5. Method according to any of claims 1 to 4, wherein, before bringing the carbonatable material into contact with the CCh-containing gas, the granular material’s moisture content is adjusted to 5 to 12 mass%, preferably 7 to 10 mass%, and in particular 7.5 to 8.5 mass%.

6. Method according to any of claims 1 to 5, wherein the combined granular material is composed of at least 50 mass%, preferably at least 60 mass% of granules having a grain size of less than 0.25 mm.

7. Method according to any of claims 1 to 6, wherein the combined granular material is composed of at least 40 mass%, preferably at least 45 mass% of granules having a grain size of less than 0.125 mm.

8. Method according to any of claims 1 to 7, wherein the combined granular material is composed of granules having a maximum grain size of 1.5 mm.

9. Method according to any of claims 1 to 8, wherein the combined granular material is composed of at least 50 mass%, preferably at least 60 mass%, and in particular at least 70 mass% of granules obtained from slag originating from steel manufacturing, in particular stainless-steel manufacturing.

10. Method according to any of claims 1 to 9, wherein the combined granular material is compacted in the moulding phase before the carbonatable material is exposed to the CCh-containing gas, applying a compaction pressure of at least 4 MPa on the combined granular material.

11. Method according to any of claims 1 to 10, wherein the combined granular material is compacted in the moulding phase before the carbonatable material is exposed to the CCh-containing gas, applying a compaction pressure of 4 to 9 MPa on the combined granular material, preferably of 4 to 8 MPa, and in particular of 4.5 to 8 MPa.

12. Method according to any of claims 1 to 11, wherein the CO2- containing gas contains at least 5 volume% of CO2, preferably at least 10 volume% of CO2, and in particular at least 20 volume% of CO2.

13. Method according to any of claims 1 to 12, wherein the magnesium nitrate is added as an aqueous solution.